Masashi Arakawa, Tsubasa Omoda, Akira Terasaki, Adsorption and Subsequent Reaction of a Water Molecule on Silicate and Silica Cluster Anions, Journal of Physical Chemistry C, 10.1021/acs.jpcc.6b11689, 2017.01, We present reactions of size-selected free silicate, MglSiOm−, and silica, SinOm−, cluster anions with a H2O molecule focusing on H2O adsorption. It was found that H2O adsorption to MglSiOm− with l = 2 and 3 (m = 4−6) is always followed by molecular oxygen release, whereas reactivity of the clusters with l = 1 (m = 3−5) was found to be much lower. On the contrary, in the reaction of SinOm− (n = 3−8, 2n − 1 ≤ m ≤ 2n + 2), a H2O adduct is observed as a major reaction product. Larger and oxygen- rich clusters tend to exhibit higher reactivity; the rate constants of the adsorption reaction are 2 orders of magnitude larger than those of CO adsorption previously reported. DFT calculations revealed that H2O is dissociatively adsorbed on SinOm− to form two SiO3(OH) tetrahedra. The site selectivity of H2O adsorption is governed by the location of the singly occupied molecular orbital (SOMO) on SinOm−. The present findings give molecular-level insights into H2O adsorption on silica and silicate species in the interstellar environment..

Masashi Arakawa, Kei Kohara, Akira Terasaki, Reaction of aluminum cluster cations with a mixture of O2 and H2O gases: Formation of hydrated-alumina clusters, Journal of Physical Chemistry C, 10.1021/jp511293g, 119, 10981-10986, 2015.04, We present reactions of size-selected free aluminum clustercations, AlN+ (N = 1−14), exposed to a mixture of water and oxygen gases.It is featured that chemical species assignable to Al2O6H7+ and Al2O7H9+were commonly produced as prominent reaction products from all of thesizes, except N = 1. These product ions were found to be produced via theformation of Al2O3+ in the initial stage of reactions with O2 and H2O,which was followed by successive hydrogenation and hydration. Thisreaction pathway was identified by examining reactivity of eachintermediate product step by step. Structures of the product ions wereanalyzed by collision-induced dissociation experiments and DFT calculations; for+ example, coexistence of isomers, Al2O5H5(H2O)+ andAl2O4H3(H2O)2 , with one and two intact H2O molecules, respectively, was suggested for Al2O6H7+. The chemical compositions of the ions +produced in the present reactions are expressed nominally as Al2O3(H2O)nH , which is similar, except for the proton, to that of hydrated alumina, that is, forms of bulk aluminum abundant naturally. The present finding gives molecular-level insights into formation processes of aluminum minerals in a natural environment..